The now ubiquitous telecommunication instruments commonly called cellular telephones (or simply “cell phones”) are actually mobile radios having a transmitter and a receiver, a power source, and some sort of user interface. They are referred to as cell phones because they are designed to operate within a cellular network. Despite being radios, they typically do not communicate directly with each other. Instead, these mobile telephones communicated over an air interface (radio link) with numerous base stations located throughout the network's coverage area. The network base stations are interconnected in order to route the calls to and from telephones operating within the network coverage area.
FIG. 1 is a simplified block diagram illustrating the configuration of a typical cellular network 100. As may be apparent from its name, the network coverage area (only a portion of which is shown in FIG. 1) is divided into a number of cells, such as cells 10 through 15 delineated by broken lines in FIG. 1. Although only six cells are shown, there are typically a great many. In the illustrated network, each cell has associated with it a base transceiver station (BTS). Generally speaking, BTS 20 is for transmitting and receiving messages to and from any mobile stations (MSs) in cell 10; illustrated here as MS 31, MS 32, and MS 33, via radio frequency (RF) links 35, 36, and 37, respectively. Mobile stations MS 31 through MS 33 are usually (though not necessarily) mobile, and free to move in and out of cell 10. Radio links 35-37 are therefore established only where necessary for communication. When the need for a particular radio link no longer exists, the associated radio channels are freed for use in other communications. (Certain channels, however, are dedicated for beacon transmissions and are therefore in continuous use.) BTS 21 through BTS 25, located in cell 11 through cell 15, respectively, are similarly equipped to establish radio contact with mobile stations in the cells they cover.
BTS 20, BTS 21, and BTS 22 operate under the direction of a base station controller (BSC) 26, which also manages communication with the remainder of network 100. Similarly, BTS 23, BTS 24, and BTS 25 are controlled by BSC 27. In the network 100 of FIG. 1, BSC 26 and 27 are directly connected and may therefore route calls directly to each other. Not all BSCs in network 100 are so connected, however, and must therefore communicate through a central switch. To this end, BSC 20 is in communication with mobile switching center MSC 29. MSC 29 is operable to route communication traffic throughout network 100 by sending it to other BSCs with which it is in communication, or to another MSC (not shown) of network 100. Where appropriate, MSC 29 may also have the capability to route traffic to other networks, such as a packet data network 50. Packet data network 50 may be the Internet, an intranet, a local area network (LAN), or any of numerous other communication networks that transfer data via a packet-switching protocol. Data passing from one network to another will typically though not necessarily pass through some type of gateway 49, which not only provides a connection, but converts the data from one format to another, as appropriate.
Note that packet data network 50 is typically connected to the MSC 29, as shown here, for low data rate applications. Where higher data rates are needed, such as in 1xEV-DO or 1 xEV-DV networks, the packet data network 50 is connected directly to the BSCs (26, 27), which in such networks are capable of processing the packet data.
The cellular network 100 of FIG. 1 has several advantages. As the cells are relatively small, the telephone transmitters do not need a great deal of power. This is particularly important where the power source, usually a battery, is housed and carried in the cell phone itself. In addition, the use of low-power transmitters means that the mobile stations are less apt to interfere with others operating nearby. In some networks, this even enables frequency reuse, that is, the same communication frequencies can be used in non-adjacent cells at the same time without interference. This permits the addition of a larger number of network subscribers. In other systems, codes used for privacy or signal processing may be reused in a similar manner.
At this point, it should also be noted that as the terms for radio telephones, such as “cellular (or cell) phone” and “mobile phone” are often used interchangeably, they will be treated as equivalent herein. Both, however, are a sub-group of a larger family of devices that also includes, for example, certain computers and personal digital assistants (PDAs) that are also capable of wireless radio communication in a radio network. This family of devices will for convenience be referred to as “mobile stations” (regardless of whether a particular device is actually moved about in normal operation).
In addition to the cellular architecture itself, certain multiple access schemes may also be employed to increase the number of mobile stations that may operate at the same time in a given area. In frequency-division multiple access (FDMA), the available transmission bandwidth is divided into a number of channels, each for use by a different caller (or for a different non-traffic use). A disadvantage of FDMA, however, is that each frequency channel used for traffic is captured for the duration of each call and cannot be used for others. Time-division multiple access (TDMA) improves upon the FDMA scheme by dividing each frequency channel into time slots. Any given call is assigned one or more of these time slots on which to send information. More than one voice caller may therefore use each frequency channel. Although the channel is not continuously dedicated to them, the resulting discontinuity is usually imperceptible to the user. For data transmissions, of course, the discontinuity is not normally a factor.
Code-division multiple access (CDMA) operates somewhat differently. Rather than divide the available transmission bandwidth into individual channels, individual transmissions are spread over a frequency band and encoded. By encoding each transmission in a different way, each receiver (i.e. mobile station) decodes only information intended for it and ignores other transmissions. The number of mobile stations that can operate in a given area is therefore limited by the number of encoding sequences available, rather than the number of frequency bands. The operation of a CDMA network is normally performed in accordance with a protocol referred to as IS-95 (interim standard-95) or, increasingly, according to its third generation (3G) successors, such as those sometimes referred to as 1xEV-DO and 1xEV-DV, the latter of which provides for the transport of both data and voice information.
FIG. 2 is a flow diagram illustrating the basic steps involved in sending a CDMA transmission according to the prior art. At START it is assumed that information from an information source (such as a caller's voice) is available and that a connection has been established with a receiving node. At step 205, the audible voice information is sampled and digitally encoded. The encoded information is then organized into frames (step 210). Error detection bits are then added (step 215) so that the receiver can evaluate the integrity of the received data. The resulting signal is then convolutionally encoded (step 220). Block interleaving is then performed (step 225) on the resulting signal to further enhance the receiver's ability to reconstruct the bit stream with a minimum of error. The interleaved signal is then spread by a pseudonoise (PN) code (step 230), a long code is applied (step 235) and a Walsh code is used to spread the wave form and provide channelization (step 240). I and Q short codes are added (step 245) and the results filtered (step 250) before being combined and spread (step 255), then amplified (step 260) for transmission.
As alluded to above, mobile stations and the network they are a part of are presently being used to carry an increasingly large amount of traffic. Not only is the number of ordinary voice calls increasing, but so is the number of other uses to which mobile stations can be put. Short message service (SMS) messaging and instant messaging are becoming more popular, faxes and emails can be sent through mobile stations, and World Wide Web pages can be downloaded. Portable personal computers can be equipped to send through the network data files such as spreadsheets, word processing documents, and slide presentations. All of this information may enter and leave the network infrastructure through the air interface, meaning that more efficient methods of radio transmission are constantly in demand. The present invention presents a solution that addresses this growing need.